On a chilly October night at 57 West 75th Street, better known to local Manhattan residents as the La Rochelle Bldg. police hovered around the luxury apartment building where apparently a nanny stabbed two children she was in charge of caring for, to death Thursday. A Manhattan nanny was later taken into custody Thursday night after she was discovered near the bodies of two of her young charges, who had been fatally stabbed, police said.

The children, identified by a law-enforcement official as 1-year-old Leo Krim and his 6-year-old sister, Lucia, were found by their mother after she returned home about 5:30 p.m. They were both taken to St. Luke’s Hospital, where they were declared dead.

Yoselyn Ortega, 50, who had been watching the children, was lying unconscious on the bathroom floor with apparent slash wounds to her neck and a kitchen knife by her side, police said. She is believed to have attempted suicide. “That is the operating premise, right now, that they were self-inflicted,” New York Police Commissioner Raymond Kelly said at a news conference after an unrelated event in Midtown.

Ms. Ortega was also taken to St. Luke’s, where she was in critical but stable condition. The official said investigators believe she committed the murders, but she had not yet been formally charged at press time.

It is said that the mother of the two youngsters, Marina Krim, arrived at the 11-story home with another child, a 3-year-old daughter, she found the second-floor apartment dark, police said. She then went to the lobby to ask the doorman if Ms. Ortega had taken the children out, police said.

The doorman told Ms. Krim they had not, and Ms. Krim returned to the apartment where she found the children and Ms. Ortega in the bathroom. The children were in the bathtub, Mr. Kelly said.

The 36-year-old mother had been to swimming lesson with her surviving daughter, Mr. Kelly said. Her husband, media executive Kevin Krim, 37, was traveling out of state and wasn’t home at the time of the killing. The devastated father returned home on Thursday night.

The mother’s screams were heard by a neighbor who called 911, police said. Arriving officers discovered the children with multiple stab wounds, according to Paul Browne, a spokesman for the New York Police Department. Neighbors describe Mrs. Krim as “inconsolable, hysterical an frantic.”

A New York City Fire Department official said the mother was taken to St. Luke’s, where she was being treated for trauma. Late on Thursday, detectives were with people in the Hamilton Heights apartment where Ms. Ortega lives.

Fernando Mercado, 56, is the superintendent of the building Ms. Ortega lives in. He said Ms. Ortega lives with her sister and niece and has lived in the apartment for about 20 years. Mr. Kelly said the investigation was still in a very preliminary stage late on Thursday and it was unclear how long Ms. Ortega had been with the Krim family.